FSK is a method used for data modulation, which has a logical 1 (a mark) designated by a carrier signal modulated by a low frequency f.sub.L, and a logical 0 (a space) designated by the carrier signal modulated by a higher frequency f.sub.H. In North America the lower frequency signal has been standardized for telephony at 1200+/-12 Hz, and the higher frequency signal at 2200+/-22 Hz.
While being carried over telephone trunks, the signal at the higher frequency suffers from attenuation that is different from and is usually higher than that of the signal at the lower frequency. This results in an imbalance between the levels of the signals at the lower and higher frequencies. This imbalance is called "twist", and is observed at the input of an FSK receiver. It is imperative that an FSK receiver copes well with signals suffering from twist as severe as e.g. 6 dB. FSK receivers typically utilize baud clock recovery circuits and AGC circuits to deal with the deleterious effects of twist.